Curing technology involving activation energy rays such as electron beams and ultraviolet rays is important in the fields of adhesives, coating agents, encapsulating materials, inks and sealing materials from the viewpoints of recent organic solvent emission control and reduction of consumption of production energy. The adhesives, coating agents, encapsulating materials and inks used in precision parts such as electric and electronic parts are most often hygroscopic and contain water, so that their use in water-hating precision parts is generally difficult. Even if the use is possible, the following problem is encountered: the precision parts are subjected to elevated temperatures of 200° C. or above in the soldering or other processing, so that the water contained in the layer of adhesive, coating agent, encapsulating material or ink is rapidly evaporated to cause internal stress, resulting in cracks in the layer of adhesive, coating agent, encapsulating material or ink. This evaporation of water in the coating agents, inks and sealing materials is also a cause of cracks and separation of layers in the fields of coating agents, inks and sealing materials for use in other than precision parts. Accordingly, there has been an increasing need for a material having flexibility, high elongation, excellent adhesion to metals and good waterproofness.
The curing technologies in the above-mentioned fields are largely classified into: radical photocuring technology in which a monomer or an oligomer being a polyfunctional acrylate or an unsaturated polyester is cured with an activation energy ray such as UV ray in the presence of a radical photopolymerization initiator, or radical heat curing technology in which the above monomer or oligomer is cured by heat in the presence of a thermally decomposable radical polymerization initiator; cationic photocuring technology in which a monomer or an oligomer being an epoxy compound or an oxetane compound is cured with an activation energy ray such as UV ray in the presence of a cationic photopolymerization initiator; and heat curing technology in which the above monomer or oligomer is cured by heating in the presence of an acid anhydride, an amine compound, a phosphine compound or a phenolic resin.
The radical photocuring technology has characteristics of a high curing rate of composition and a wide variety of applicable monomers and oligomers to permit preparation of cured products with various properties. However, the polymerization is easily inhibited by air oxygen, and the monomers and oligomers used are highly toxic and possess strong odor and skin irritation.
Characteristics of the cationic photocuring technology include small shrinkage of a composition cured with an activation energy ray such as UV ray, and cured products having good adhesion with metals. However, the curing rate is low and the variety of monomers and oligomers employable is limited.
The radical heat curing and the heat curing technologies can cure deep inside a composition that the activation energy rays such as UV rays do not reach, but they have a problem of short pot life.
Accordingly, it has been difficult that a curable composition is cured by any single technology of the aforesaid while satisfying two or more of the characteristics of a high curing rate, small shrinkage when cured, and good adhesion with metals.
To solve the above problems, means proposed include (1) a photocurable resin composition for sealing that essentially contains a resin whose main chain skeleton is composed of a butadiene homopolymer or a butadiene copolymer and which has an average of at least 1.5 epoxy groups in a molecular terminal and/or a side chain per molecule, an epoxy resin, an acrylate or a methacrylate, and a photosensitive aromatic onium salt (JP-A-S61-51024); (2) a resin paste composition that contains an acrylate or a methacrylate compound, an epoxidized polybutadiene, a radical initiator, and a filler (JP-A-2000-104035); and (3) an insulating resin composition for multilayer printed wiring board that contains at least: a photocurable resin being a UV curable resin that is obtained by reaction of a reaction product between a bisphenol epoxy resin compound and an unsaturated monocarboxylic acid, with a saturated or unsaturated polybasic acid anhydride; a thermosetting component being a polyfunctional epoxy resin; a polybutadiene in which part of double bonds remaining in the molecule is epoxidized; an epoxy compound having a photocurable component and a thermosetting component; a photopolymerization initiator; and a filler, wherein the insulating resin composition is developable by a dilute alkaline solution and has photocurable and thermosetting properties (JP-A-H11-214813).
The compositions of (1) to (3) have a common technical idea that flexibility is imparted by the epoxy-modified polybutadiene and the curable resin compositions are curable by two or more curing technologies in combination to achieve combined characteristics of these technologies. The curable compositions disclosed possess good adhesion and high curing rate. The resin whose main chain skeleton is composed of a butadiene homopolymer or a butadiene copolymer and which has an average of at least 1.5 epoxy groups in a molecular terminal and/or a side chain per molecule, which is disclosed to be substantially useful in (1), is for example an epoxidized polybutadiene (Examples disclose a molecular weight of 1500 and an epoxy oxirane oxygen content of 7.7% (corresponding to an epoxy number of 4.8 meq/g)) that is highly modified or contains an epoxy group at a molecular terminal. The epoxidized polybutadiene, which is disclosed to be substantially useful in (2), has a preferable epoxy equivalent of 50 to 500 (corresponding to an epoxy number of 20 to 2 meq/g) and is highly modified (Examples disclose epoxy equivalents of 152.4 to 177.8 (corresponding to epoxy numbers of 6.6 to 5.6 meq/g)). The polybutadiene in which part of double bonds remaining in the molecule is epoxidized, which is disclosed to be substantially useful in (3), has a preferable epoxy equivalent of 150 to 250 (corresponding to an epoxy number of 6.7 to 4 meq/g) and is highly modified.
When such highly epoxidized polybutadienes are cured, the crosslink density is increased and the cured products cannot display adequate flexibility. Further, the epoxy-terminated epoxidized polybutadiene, which is generally produced by reaction with epichlorohydrin, contains large amounts of impurities such as by-product chloride ions, so that the epoxy resin composition shows lowered humidity resistance and exhibits a corrosive action when used in contact with metal parts.
It is therefore an object of the present invention to provide a curable composition that shows high elongation and excellent rubber elasticity even in a cured state and has superior compatibility, transparency, flexibility and waterproofness.